Chromogenic anion sensors
Chromogenic sensors for anions generally consist of two parts: anion receptors and chromophores. In this review, 6 types of chromogenic anion sensors are described, namely, NH-based hydrogen bonding, Lewis acid, metal-ion template, transition metal complexes, chromogenic guest displacement and chromoreactands. The first 4 types possess anion receptors attached directly to the chromophores while the guest displacement techniques employ indicators as the ones that were replaced by specific anions. The last type has emerged recently and uses specific reactions between chromogenic hosts or indicators and particular anions to cause dramatic colour changes.